The vertebrate kidneys themselves are varied in structure; the ducts vary as does the urinary bladder. These variation are due to two major causes: (l) The kidney unlike many organs must Start fiJnctioning at an early Stage Of development in order to remove the metabolic of the rapidly growing embryo. The kidney is however, subject to modification or replacement in the later developmental Stages and adult existence (ii) the gonads (testis and ovary) lie adjacent to the kidneys. These organs specially the testis tend to take over part of the tubes and tubules of the urinary structures as a conducting system for their products. AS a result of this the urinary organs have been markedly modified in most vertebrate groups. In this subsection we will survey the construction and hence normal variations of urinogenital systems in various vertebrate groups.

Agnatha (Cyclostöme) or Jawless Vertebrates

The adult kidney of jawkss vertebrates lamprey (Petryomyzon) is opisthonephros or Mesonephrons andthe tubules are derived from the posterior mesomere. The tubules have lost all connection with the coelomic cavity but form a close association with the glomerular blood vessels at the blind-ended kidney capsule. The absence of connection between kidney tubules and coelom makes it evident that "coelomic fluid filter" was replaced by "blood! filter system" early in vertebrate evolution. This may be because possibly coelomic fluid pressure is too low and variable to keep up with the fluid balance demands of vertebrates which have high metabolic rates.

In adult lamprey the kidney is elongated and flattened. The kidneys lie on either side or the mid-dorsal line and each is suspended by a mesentery like membrane. The archinephric duct courses along the free edge of the kidney The pronephros exists above the mesonephros as the non functional "head kidney"

In the adult hagfish the opisthonephrös consists simply Of a series of tubule arranged in a segmental manner along most Of the length of the trunk, each tubule draining directly into the archinephric duct or pronephros. In hagfish adults the archinephros becomes modified to form a persistent head kidney. The remainder of the kidney, however is separated from the head kidney and becomes the opisthonephros. The hagfish has only forty glomeruli, each connected to the collecting duct (archinephric duct) by a small neck Segment.

Fish

The kidneys of fishes are opisthonephric or mesonephric and though they exhibit great variation in shape. they are however fundamentally similar in structure  In all species they are dorsal in position. In some they extend almost the entire length Of In Other fishes they may be more voluminous and the two sides may show various degree of füsion. In still others they are short and confined to the posterior pan Of the body cavity. Peritoneal funnels are only retained in a few forms, notably Amia, Sturgeons and certain elasmobranchs. In some marine teleosts no external or internal glomeruli are present. and so such kidneys are called as aglomerular kidneys.

Generally the kidneys Of male fish are longer than those Of females, since the anterior ends in male are appropriated by the reproductive system. In males Of some groups small modified kidney tubules now called efferent ductules, connect the testes with the archinephric duct. This archinephric duct is termed as the ductus deferens and serves as the passage for sperm transport. It may, in addition continue to carry wastes. However, in such cases there is a marked tendency for the posterior